This invention relates to plasma processing of a substrate and, more particularly, to an apparatus and method for improving the plasma density in a plasma processing system.
The fabrication of semiconductors, or integrated circuits, typically consists of multiple distinct processing steps, during which numerous replicas of an integrated circuit are formed on a single substrate or wafer. Generally, the fabrication procedure entails the creation of multiple patterned layers on and into the surface of the semiconductor substrate, ultimately forming the electrically active regions, passive elements and interconnects that comprise the integrated circuit.
Modern techniques for fabricating integrated circuits liberally incorporate plasma processes to modify the surface of the semiconductor substrate and to generate the multiple patterned layers. For example, a plasma etching process employs a plasma to selectively remove a layer of extraneous material from the substrate surface or to condition the surface by low-energy ion bombardment. As another example, a physical vapor deposition process may employ a plasma to control the characteristics of the thin films deposited onto the substrate surface from a source of coating material. A high degree of ionization of neutral atoms of coating material has been found to enhance the control over the coating material in transit towards the substrate.
Ionized physical vapor deposition (IPVD) is a plasma-enhanced deposition process used to deposit a thin film or coating onto the surface of the semiconductor substrate. A source of a coating material is positioned within a vacuum chamber usually opposite a substrate support holding the substrate. The source generates a flux of atoms or atomic clusters of coating material, such as by the sputtering of a solid target having the desired elemental composition. To deposit as a thin film upon the substrate, the flux of coating material must traverse a high-density plasma confined within a processing space separating the source and the substrate. The high-density plasma may be generated by coupling radio-frequency (RF) energy to a process gas maintained at an operating pressure in the processing space.
A significant fraction of the flux of coating material is ionized by collisional interactions with the positive ions of process gas (Penning ionization) and free electrons (impact ionization) constituting the high-density plasma. At high density plasma where the electron density exceeds 1011 cmxe2x88x923, electron impact ionization is the dominant process in IPVD. To preferentially attract positive ions of coating material, the substrate may be negatively biased. The negative bias potential may arise incident to immersion of an ungrounded substrate in the plasma if the substrate support is electrically floating or by directly applying a bias voltage to the substrate support and substrate. The negative bias potential accelerates and steers the trajectories of the positive ions of coating material such that the ions tend to strike the surface of the substrate with a near-normal angle of incidence. As a result, the deposited coating material will more effectively cover the bottoms and sidewalls of submicron features having a high aspect ratio, such as vias, lines, contact holes, and trenches.
The RF energy for generating the high-density plasma is supplied by an RF power supply operably connected to an antenna or excitation coil positioned either external or internal to the vacuum chamber. If the excitation coil is externally positioned, a wall of the vacuum chamber may be further provided with a dielectric window which permits RF energy from the coil to ignite and sustain the plasma and isolates the coil from direct contact with the plasma.
The dielectric window is typically masked by an electrostatic shield, typically formed of an electrically-conductive material and disposed in the vacuum chamber, which functions as a Faraday shield and as a physical shield. A plurality of openings in the shield permit inductive coupling of RF energy emanating from the excitation coil with the plasma while suppressing the unwanted component of parasitic capacitive coupling. As a physical shield, the shield prevents an unwanted conductive layer of coating material from depositing onto the window by concealing the window from the plasma. If an electrically conductive layer of coating material deposits on the window, RF energy from the excitation coil can no longer couple efficiently with the plasma since inductive RF field is absorbed exponentially with the penetration depth in a conductive material. As a result, the plasma density will be reduced and the deposition process deteriorates or the RF power must increased to compensate for the reduced density. If thickness of the unwanted conductive layer exceeds a frequency-dependent threshold, called the skin depth, then significant RF power loss will occur.
A typical coating material source sputters a target, composed of high purity coating material, that is negatively biased with respect to plasma confined close to the target and a chamber anode such as the grounded wall of the vacuum chamber. Usually, the target is operably connected to a direct current power supply that supplies a bias potential for attracting positive ions from the high-density plasma. The source is frequently of a magnetron design which incorporates a magnet structure for creating and confining plasma adjacent the target.
Conventional inductively-coupled plasma processing systems have shortcomings and deficiencies that restrict their widespread application for large-area wafer processing. High-density plasmas generated by inductively coupled plasma generating assemblies exhibit significant radial non-uniformities in plasma density. Due to losses near the chamber walls, the plasma in the processing space has a density distribution that is preferentially peaked about the central symmetry axis of the vacuum chamber and depleted of positive ions near the chamber walls. If such a radially non-uniform plasma is used for etching the surface of a substrate, the removed layer will be thinner near the periphery of the substrate due to reduced ion flux that controls etching rate. In an IPVD apparatus, a radially non-uniform distribution in plasma density may affect the properties of the deposited thin film and coverage of features. For example, the thin film thickness may be uniform due to the target geometry, but step coverage may vary across the wafer diameter due to non-uniformities in the ion distribution. Non-uniformities in etching or deposition are most pronounced for substrates having larger diameters. Since the trend in semiconductor fabrication is toward large-area wafers, the presence of non-uniformities in the plasma density will be more significant in future plasma processing systems, such as IPVD systems and plasma etching systems.
Plasmas generated by inductively coupled plasma generating assemblies exhibit certain limitations regarding the amount of RF power that must be supplied to initiate an inductively-coupled, high-density plasma. Under certain circumstances, the inductively coupled plasma must be extinguished by reducing the RF power. For example, the RF power must be reduced to load or unload substrates from the vacuum chamber or the RF power supply rendered inoperative if the vacuum chamber is vented to atmosphere pressure. To reinitiate the inductively-coupled component of the plasma, a large amount of RF power must be provided by the RF power supply. For example, the power needed to initiate an inductively-coupled high-density plasma may exceed the power needed to initiate a capacitively-coupled plasma by an order of magnitude, under similar chamber conditions.
As a result of the above considerations and problems, there remains a need for an apparatus and method that can supplement the primary, high-density inductively-coupled plasma of an inductively-coupled plasma processing system for increasing plasma uniformity adjacent the substrate and for reducing the RF power required to initiate an inductively-coupled plasma.
The present invention advantageously provides an apparatus and a method for improving the uniformity of the plasma density in an inductively-coupled plasma processing system. The present invention further advantageously provides an apparatus and a method in which a supplemental capacitively-coupled plasma is provided so that a reduced RF power level is required to initiate a high-density, inductively-coupled plasma in a processing system. Moreover, the present invention advantageously provides a more efficient and effective apparatus and method for plasma processing operations that can be incorporated into current plasma processing systems without significantly altering conventional chamber designs.
According to the principles of the present invention, one or more hollow anode assemblies are located about the interior of the vacuum chamber of a plasma-processing apparatus that relies upon an inductively-coupled, high-density plasma for processing a substrate. Each hollow anode assembly comprises one or more enclosures or discharge cavities which receive a portion of a process gas residing in the vacuum chamber and which are operable for containing a capacitively-coupled plasma therein. Positive ions of process gas and electrons from the capacitively-coupled plasma exit each cavity through one or more outlets provided therein and enter the vacuum chamber.
The vacuum chamber further includes a plasma generating assembly that is configured to deliver energy into the vacuum chamber for generating the inductively-coupled, high-density plasma. An exemplary plasma generating assembly includes an RF power supply operably connected to an excitation coil, which can also provide the energy to initiate and sustain the capacitively-coupled plasma in each cavity.
Enclosures may be positioned, for example, adjacent to the plasma generating assembly or adjacent to a substrate support. However, in positions with the vacuum chamber remote from the plasma generating assembly, the coupled energy may be insufficient to generate the capacitively-coupled plasma within each cavity or the excitation coil would require modifications which would add complexity to the processing system. In these cases, the plasma generating assembly may further incorporate an ancillary power supply operable to generate the capacitively-coupled plasma. In one aspect, an electrode may be positioned within the cavity of each enclosure and operably connected to the ancillary power supply.
According to the apparatus and method of the present invention, one advantage is that a capacitively coupled plasma can be initiated and sustained in the cavity of each enclosure without the need for an additional plasma generating assembly. Power from the plasma generating assembly, which is principally operable for inductively coupling with process gas in the processing space, can capacitively couple with process gas to generate a plasma in each enclosure. It follows that the enclosure, in certain embodiments, can be a passive element of the plasma processing system without requiring an electrode, an additional power supply, or an electrical feedthrough.
Another advantage of the apparatus and method of the present invention is that the density distribution of the inductively coupled plasma in the processing space can be efficaciously modified by emitting ions and electrons from each enclosure and, as a result, the plasma process will achieve results having improved uniformity.
Yet another advantage of the apparatus and method of the present invention is that the power required to initiate an inductively coupled plasma in the processing space can be significantly reduced by providing electrons and ions from the capacitively coupled plasma generated within each enclosure prior to ignition of an inductively-coupled plasma in the processing space.
The present invention may be incorporated into an existing processing chamber merely by modifying the shield structure to add one or more enclosures. Therefore, the addition of one or more of the enclosures will not significantly alter design of the processing chamber, while optimizing plasma processing and overcoming the difficulties set forth in the background above.
These and other advantages of the present invention will be more readily apparent from the following detailed description of the drawings.